GFCI devices are designed to trip in response to the detection of a ground fault condition at an AC load. Typically, the ground fault condition results when a current path is provided between the hot or neutral conductor and earth ground, such as when a person comes into contact with the line side of the AC load and an earth ground at the same time. Such a condition is dangerous and can result in serious injury. Conventional GFCI devices detect this condition by using a sensing transformer to detect an imbalance between the currents flowing in the line and neutral conductors of the AC supply, as will occur when some of the current from either the hot or neutral conductor is being diverted to ground. When such an imbalance is detected, a solenoid activates a latched circuit breaker within the GFCI device to an open condition, thereby opening both sides of the AC line and removing power from the load.
Conventional GFCI devices provide a manual test mechanism. Typically, a manual switch is provided on the face of the GFCI receptacle. When a user presses the switch, current is diverted from the line side to the load side around the sensing transformer, resulting in a current imbalance through the transformer which is sensed. If the GFCI is functioning properly, the ground fault detecting circuit detects the output of the sensing transformer and causes the circuit breaker to open and a reset button to extend from the receptacle face. The user can press the reset button back into the receptacle face to reset the circuit breaker and reconnect the contacts. Users are encouraged to perform a manual test at least once per month, and to replace the receptacle if the unit fails to trip. Unfortunately, many users forget to perform a manual test each month. Accordingly, if any of the components of the GFCI device fail, the user may not be aware that ground fault protection is no longer being provided, and that the GFCI device should be replaced.
More recently, self testing GFCI devices have been provided which periodically and automatically test various components of the device to verify, for example, that the sensing transformer provides an output to the ground fault detecting circuit if an imbalance between hot and neutral conductors exists, that the ground fault detecting circuit provides an output to the circuit breaker tripping device, and that the relay coil of the circuit breaker is not an open circuit (i.e. a broken conductor). This alleviates the problem of users not remembering to manually test the GFCI device. Unfortunately, conventional self testing GFCI devices suffer from other problems. Some designs trip the circuit breaker periodically, causing interruption of power to the load. This is not suitable for use with load equipment which requires continuous power, such as medical equipment, for example.
Other self-testing GFCI devices are designed to test the sensing transformer, the ground fault detecting circuit, the tripping device or the relay coil without actually tripping the circuit breaker and causing an interruption of power to the load. Unfortunately, conventional self testing GFCI devices suffer from disadvantages in that they are unable to detect a failure of the contacts to open if the tested components are functioning properly, such as if the contacts are welded together. Futhermore, such devices may be unable to open the contacts even when a failed component is identified, such as if the tripping device is short circuited.
U.S. Pat. No. 6,262,871 to David C. Nemir, et al., the entire contents of which are hereby incorporate by reference, provides a GFCI device that tests whether the circuit breaker contacts actually open or not, and activates a secondary circuit breaker if the first circuit breaker fails. This provides failsafe operation in the even that the first circuit breaker fails, but the second circuit breaker is still working. Unfortunately, this solution adds components, complexity and cost to the GFCI device, and as a result, users may select other, cheaper models.
Accordingly, there is a need for a manual test GFCI device which senses an end-of-life condition of the GFCI, such as if the contacts fail to open after a ground fault is sensed, and activates a secondary circuit to: pen the contacts if the primary tripping mechanism fails.